Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a visible record of the electrostatic image.
The electromagnetic radiation used to produce the electrostatic latent image on the photoconductive element can come from a variety of sources. For example, optical exposure or electronic exposure using a laser scanner or light-emitting diode array can be used. In certain cases, it is desirable to use illumination of specific wavelength ranges for producing the electrostatic latent image. For example, reproduction of color images may require the use of an illumination source or exposure means that employs filters that limit the wavelengths of illumination reaching the photoconductive element in order to allow separation of the colors of the image. In certain cases, it is desirable that some or all of the illumination be in the wavelength range of 350 to 500 nanometers (nm), the blue region of the spectrum. Exposure by light with these wavelengths may occur when a filter is used to give blue light passage for a color separation process in producing color images or when a blue laser is used as the illumination source, for example. It is desirable to have an electrophotographic apparatus that uses exposures in the blue region of the spectrum.
Photoconductive elements useful in electrophotographic apparatuses must be sensitive to the wavelengths of illumination reaching them. In particular, a photoconductive element must display good photosensitivity. Photosensitivity is a measure of the amount of energy that must be supplied during exposure to discharge the element in an image-wise fashion. For high process efficiency, high photosensitivity and low energy requirements for discharge are desired.
An important group of photoconductive elements used in electrophotographic imaging processes comprise a conductive support in electrical contact with a charge generation layer (CGL) and a charge transport layer (CTL). A CGL is designed primarily for the photogeneration of charge carriers (holes and electrons). A CTL is designed primarily for transportation of the generated charge carriers. The combination of all CGLs and CTLs in a photoconductive element is sometimes referred to as the photoconductive layers. Electrophotographic elements having one CGL and one CTL are sometimes referred to as dual layer photoconductive elements. Representative patents disclosing methods and materials for making and using such elements include U.S. Pat. No. 5,614,342 to Molaire et al.; U.S. Pat. No. 4,175,960 to Berwick et al. and U.S. Pat. No. 4,082,551 to Steklenski et al.
Photoconductive elements containing two or more CGLs and at least one CTL, referred to herein as multilayer photoconductive elements, are known. Photoconductive elements containing a CTL and two CGLs were disclosed in U.S. Pat. No. 5,213,927 by Kan et al. This patent shows that the inclusion of two CGLs, the first containing a charge-generation material and a first charge-transport material, and the second containing a second charge transport material that is less susceptible to positive-surface charge injection than is the first charge-transport material, gives a photoconductive element with improved charge uniformity and charge acceptance upon cycling.
Multilayer photoconductive elements frequently have protective overcoats on their outermost surface to protect from damage incurred during the electrophotographic process or during installation of the element in the apparatus. The overcoat imparts longer process lifetimes to the elements. Typical overcoat materials include diamond-like carbon (DLC) or amorphous carbon films. U.S. Pat. No. 4,965,156 to Hotomi et al. discloses the use of two protective layers on an organic photoconductive element. The first layer is an amorphous carbon layer which includes more than 5 atomic percent fluorine. The second, outermost layer is a similar material except that the fluorine content must be lower than 5 atomic percent. U.S. Pat. No. 5,525,447 to Ikuno et al. discloses an electrophotographic photoconductive element with a surface protective layer formed on the photoconductive layer. The surface protective layer is a multi-layer or graduated layer structure having at least one additive element selected from the group consisting of nitrogen, fluorine, boron, phosphorous, chlorine, bromine, and iodine. The additive element is at a higher concentration near the surface of the protective layer than at the interface between the protective layer and the photoconductive layer. When the additive element is fluorine, the fluorine to carbon atomic ratio (F/C) of 0.001 or less (less than 1% fluorine) in the vicinity of the photoconductive layer adjacent to the protective layer and of 0.005 or more in the vicinity of the top surface of the protective layer.
A problem associated with protective overcoats is the undesirable absorption of radiation at particular wavelengths. DLC protective overcoats known in the art have measurable absorption in the blue range (350 to 500 nm) of the spectrum. For optical copiers in particular, this is undesirable. A decrease in blue sensitivity of the photoconductive element, resulting from absorption by the protective overcoat, is known as "blue blindness." It results in loss of blue parts of a multi-color original image. Other colors, such as red, however, are reproduced as dark lines. The result is either unacceptable loss or change of information in a black and white copy, where blue information is reproduced as gray or is not reproduced at all, or an unacceptable change in the color balance of a color copy. This can also be detrimental in digital copier and printer applications where the protective overcoat can attenuate the exposure radiation. Both the inventions of Hotomi et al. (U.S. Pat. No. 4,965,156) and of Ikuno et al. (U.S. Pat. No. 5,525,447) require that the protective overcoat contain a layer or portion of the protective overcoat that imparts significant blue blindness to the photoconductive element.
Protective layers can also change the photosensitivity and residual voltage of the photoconductive element. This can result in loss of contrast between light and dark areas in the final image and in failure to reproduce some or all of an image. The impact of the protective layer on these properties depends on the combination of its properties, for example its light absorption at particular wavelengths or its resistivity, with the properties of the other layers, particularly the photoconductive layers, in the element. Thus, it is not obvious that a protective layer that has proven useful with one type of photoconductive element will work for all photoconductive elements.
It is not evident from the prior art how to construct an electrophotographic apparatus which uses blue light exposure with a photoconductive element having a protective overcoat.